The present invention relates to photolithographic fine patterning of a photoresist film in the preparation of, for example, semiconductor devices. More particularly, the invention relates to a photomask for photolithography which provides an improved high resolving power in the patterned photoresist film.
Most of the semiconductor devices are manufactured by the techniques of photolithographic fine patterning. The formation of a finely patterned layer of a photoresist material on a substrate is usually performed in the following manner. The surface of a substrate is first provided with a layer of a photoresist material which is then directly contacted with a photomask bearing a pattern of a masking material. The photoresist layer is irradiated through the masking material with actinic rays, such as ultraviolet light, so that the solubility behavior of the photoresist material becomes modified in the areas irradiated by the actinic rays and thereby forms a latent image which is a reproduction of the fine pattern of the photomask. When the solubility of the photoresist is increased by irradiation and becomes dissolvable by treatment with a developer liquid, the photoresist is called a positive-type. On the other hand, when the photoresist material is insolubilized by irradiation, so that the photoresist material on the unirradiated areas may be selectively dissolved away by the developer liquid, the photoresist material is called a negative-type. For either type, the developing treatment utilizes the solubility differences between the exposed and unexposed areas to form a latent image on the substrate.
In accordance with the rapidly increasing density of integration of the semiconductor devices such as LSIs and VLSIs in recent years, higher and higher resolving power is required in the patterning obtained in the above described manner reaching the so-called submicron region. As is known, the manufacturing process of most semiconductor devices, such as ICs, LSIs and the like, involves several and even up to 10 repeated photolithographic patterning steps. Each step is accompanied by a chemical etching procedure which causes an increasing surface ruggedness of the photoresist layer, so that the fidelity in the reproduction of the fine pattern of the photomask cannot be as high as desired on the photoresist layer. This gives rise to a limitation in the density of integration and decrease in the accuracy of working. Another factor for the decreased accuracy is the ruggedness on the surface of the photoresist layer which greatly disturbs the desirable direct and intimate contact between the photoresist layer and photomask laid thereon.
The above described problem in the fine patterning becomes more and more serious as the size of the substrate, i.e., the diameter of the wafer of silicon semiconductor, is increased.
The above described problem has been considered unavoidable insofar as photolithographic techniques are used for fine patterning in the manufacture of semiconductor devices.